The Study On The Production Process Of High Strength Super Fine IF Steel

In order to meet the require of weight reductions for the automobile body-in-white, save the energy cost and improve the collision security, the application of high-strength steel sheets for exposed panels can only progress further with the realization of the need for high-strength steel sheets combining excellent pressformability and sufficient coated surface quality for exposed automobile body panels. Sheets of this type with tensile strengths of up to440MPa are now available commercially.The fine grain high-strength IF steel can be obtained by refinement of the grain, increasing strength of the grain boundary, restraining the secondly working brittlement, using the precipitation strength by the finest particles, so it has excellent deep drawability (high r-values).In this paper, the hot deformation behavior and continuous cooling transformation behavior were studied on the MMS-300thermo-mechanical simulator. The hot rolling experiments were carried out by Φ450rolling mills of North-eastern University. The cold rolling experiments were carried out by Φ160reversing two-high cold rolling mill and the cold rolling plates were annealed by SX2-1. The microstructure was observed by the optical microscope and transmission electron microscope. Finally the mechanical properties were tested by tensile experiments. The main contents are given as follows:(1) The hot deformation behavior and continuous cooling transformation behavior of fine grain high-strength IF steel.(a) The single pass compression test showed that with10s-1strain rate and the amount of the true strain is0.5, the mode of the stress-strain curve is dynamic recovery, no change with temperature. The flow stresses increase with increasing of temperature; When hot deformation at900℃or950℃, the flow stresses increase with the increasing of deformation rate; The flow stresses increase with increasing of reduction as temperature and strain rate are constant.(b) The static softening behavior of fine grain high-strength IF steel was studied by double-pass compression test. The static softening rate increases with high deformation temperature and long holding time, the grain size becomes big too; Through Z-hollomen parameters, the static rate was calculated, and the static recrystallization activation energy is186.73kJ/mol, the results indicated that the recrystallization behavior of the steel in this paper could happened easily.(c) The CCT curve of fine grain high-strength IF steel was carried out; deformation condition and cooling rate have great effects on transformation point; deformation accelerates the ferrite transformation; the transition region of ferrite enlarges with increasing the cooling rate.(2) The rolling test at laboratory(a) The hot rolling process was established for fine grain high-strength IF steel at laboratory.(b) The microstructure and properties under different cold rolling reduction ratio were studied at laboratory. It is found that when the samples were annealed at the temperature of820℃, with the increasing of deformation, ferrite grains became finer, tensile and yield strength increased and elongation and r value increased firstly and then decreased; While at the temperature of850℃, ferrite grains, elongation and r value were increased firstly and then decreased, but tensile and yield strength were decreased firstly and then increased. Through the contrast of microstructure and properties under different reduction ratio at two annealing temperatures, the top value of r was reached at the reduction from77.5%to80%.(c) The microstructure and properties under different annealing process were studied at laboratory. It is found that when ferrite grains became bigger, tensile and yield strength decreased and elongation and r value increased with high annealing temperature and long holding annealing time.(d) The grain obtained by the simulator continuous annealing is fined and unformed compared with annealing. The yield and tensile strength ratio is low and r value is high under the simulator continuous annealing condition, but the elongation and n value has no difference.(3) The study on second-phase precipitations behavior(a) The fine grain high strength steel is refined compared with conventional IF steel. The microstructure of fine grain high strength steel contains very fine Nb(C, N) precipitates which size is about10～40nm and shows unique distribution by adding Nb. The very fine Nb(C, N) precipitates can restrain the grain growth and then the obtained grain is very fine.(b) A zone depleted of precipitates (PFZ:precipitate-free zone) is formed in a region neighboring the grain boundary by the effect the movement of grain boundary. The width of PFZ is above0.6μm which is reason for the low yield strength and high tensile strength with the super fined grains.(c) With the increasing of annealing temperature and heating rate, the PFZ(Precipitation Free Zone) near the ferrite grain boundary is getting more stable, the volume of PFZ is increased, and the yield strength is more low. PFZ is the dominant factor of yielding and strain-hardening behavior of fine-grain high-strength IF steel.(d) The fine grain high strength steel shows the high strength by fine grain strengthening, precipitated strengthening and solid solution sterengthening. The steel gives low yield strength and fine drawablity due to the existence of PFZ.(4) The development of rolling process(a) The suitable hot rolling process for fine grain high strength IF steel is as follows:the heating temperature around1150℃, the roughing temperature around1050℃, the finishing temperature about900℃,coiling temperature between630～650℃.(b) According to the study on fine grain high-strength IF steel’s cold-rolling and annealing experiments, the cold rolling process in this paper are follows:cold reduction is75～80%, the annealing temperature is840℃～860℃and the holding time is120～200s min. The tension strength of the steel reaches470MPa, the yield and tensile strength ratio is0.50～0.60, the work hardening index n is0.28and plastic strain ratio r=1.69, the elongation ratio is around40%.(c) With the increasing of cold reduction, ferrite grains became finer, tensile and yield strength increased, elongation and r value increased firstly and then decreased, the maximum value of r was obtained at the reduction of77.5%.(d) With the increasing of annealing temperature and the annealing time, ferrite grains became bigger, tensile and yield strength decreasing and elongation and r value increasing. But above a certain value, the ferrite grains, tensile and yield strength, elongation and r value didn’t changed, even decreased.